Big congrats! And welcome to NB!
I like both [name_f]Orla[/name_f] and [name_f]Heather[/name_f].
[name_f]Heather[/name_f] is pretty, soft and reliable and makes the trio a smart and assuring set of names. I get lovely, adventurous but also calm imagery from them together. [name_f]Heather[/name_f] is likely a little also easier for (non-Irish/non-rhotic accented) people to say.
[name_f]Orla[/name_f] is a name I often suggest on here, I find it so charming, pluckish and pleasant to say. I agree with @tori101 that the O sound ties these names together, and to me, they all have a golden yellow and brown warm tones. [name_f]Orla[/name_f] has a fresh feel to it!
I am stumped on which is pick - would depend on which vibe I was going for (reliable outdoorsy or more hip and spunky). [name_f]Hope[/name_f] these descriptions get you closer to deciding. Perhaps also nickname options (Orly/Lola/Lolly v Hattie/Hettie/Thea) - though they’d probably be two names that could avoid nicknames!
Meanwhile (and ignore all together if annoying), if the boys’ aren’t honour names, you could mix them up so you don’t have the repeat two syllable, similar letters, and N ending names for the boys - and to tie in [name_f]Heather[/name_f] or [name_f]Orla[/name_f] a little better (obv though individual names more important than “perfect” set):
[name_m]Simon[/name_m], [name_f]Heather[/name_f] & [name_u]Joseph[/name_u]
[name_m]Simon[/name_m], [name_f]Heather[/name_f] & [name_u]Jude[/name_u]
[name_m]Simon[/name_m], [name_m]Tobias[/name_m] & [name_f]Orla[/name_f]
[name_m]Simon[/name_m], [name_f]Orla[/name_f] & [name_u]Henry[/name_u]
[name_m]Simon[/name_m], [name_f]Orla[/name_f] & [name_m]Rafferty[/name_m] (Rafe/Raffa)
[name_m]Simon[/name_m], [name_m]Thomas[/name_m] & [name_f]Orla[/name_f]
[name_m]Simon[/name_m], [name_f]Heather[/name_f] & [name_u]Edward[/name_u]
[name_m]Simon[/name_m], [name_u]James[/name_u] & [name_f]Heather[/name_f]
[name_u]Mason[/name_u], [name_f]Orla[/name_f] & [name_u]Calloway[/name_u] (Cal)
[name_u]Mason[/name_u], [name_u]Bennett[/name_u] & [name_f]Heather[/name_f]
[name_u]Mason[/name_u], [name_m]Lucas[/name_m] & [name_f]Orla[/name_f]
[name_u]Mason[/name_u], [name_m]Harvey[/name_m] & [name_f]Orla[/name_f]
[name_u]Mason[/name_u], [name_f]Heather[/name_f] & [name_u]Everett[/name_u]
[name_u]Mason[/name_u], [name_f]Heather[/name_f] & [name_m]Andrew[/name_m]
Mix and match as you like - you could of course sub in /experiment with some of the girl suggestions from this thread - but just thinking the boys’ makes might be what’s making it tricky!